Few anecdotes have etched themselves on political hearts like the one Tony Blair told about a man he had met while canvassing. The man was polishing his Ford Sierra. He'd set up a business, bought a house and was doing nicely, "So I've become a Tory" he had said – and Mr Blair instantly knew the 1992 election was lost. Developments in the motor trade have seen the terminology change, but whether the archetypal family saloon driver was Galaxy woman or Mondeo man, the analysis remained unchallenged: winning parties need the support of voters who have worked hard to drag their families up into the middle class.

The claim was always more asserted than evidenced, which is just as well for a coalition that is about to test the patience of voters raising children on decent but unexceptional pay. The belt-tightening will eventually hurt the poor the most, but this is chiefly because of benefit cuts which are being implemented over several years. For families with combined incomes of £40,000 to £55,000, by contrast, austerity is coming early – on 6 April to be precise. From that date they will pay more tax and national insurance and also lose their tax credits. Today, the Institute for Fiscal Studies tots up the damage at £1,000 a year for some.

The package of changes – some inherited from Labour, some settled during last year's budget – is too complex to have got much attention. But it will overturn a received axiom of wisdom since Mrs Thatcher's day, by putting a tax on what Mr Blair called "aspiration". Three-quarters of a million workers will be pushed into the 40% tax bracket, while tax credit withdrawal will leave 175,000 working parents exposed to effective marginal rates of over 70%. In the theoretical worst case, which will be rare but no doubt apply to someone, Middle Englanders could be exposed to the 83% rate that old Labour levied on the super-rich. Factor in the removal of child benefit from higher-rate payers in 2013, and there are a group of middling professionals who could soon find they are little better-off – or even worse off – after a promotion.

Such effects will apply in a narrow income band, and since people strive for success for non-financial as well as financial reasons will not produce the sort of economic disaster that Thatcherite rhetoric suggests. There is no realistic way of forcing the rich to shoulder all of the fiscal tightening, so it is unavoidable that Middle Britain will feel the squeeze, and indeed preferable to the poor being hit even harder. Nonetheless, it is not merely Middle Britain but also some long-established rules of political play that will soon be upset. Listen out for howls of anguish on the Tory backbench as it wakes up to what is happening.